For resumption we need to rely on the "time" variable instead of
"position", as the total duration of the file might not have been
set yet.

Also simplifies the code.

close #11981

(cherry picked from commit c91715f76e29b933128f772d3c936c1d2eea8227)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>